Microwave harmonic generator including a waveguide having oppositely extending channels defining a resonant region therein



SAHAES Sheet of 2 March 4, 1969 J. N. LIND ET AL MICROWAVE HARMONICGENERATOR INCLUDING A WAVEGUIDE HAVING OPPOSITELY EXTENDING CHANNELSDEFINING A RESONANT REGION THEREIN Filed March 10, 1967 INVENTORS JAMESN. LIND JERRY C. AUKLAND AT TORNEY 3,431,45 MICROWAVE HARMONIG GENERATORINCLUDING A WAVEGUIDE HAVING Z of Sheet March 4, J. N. LIND ET ALOPPOSITELY EXTENDING CHANNELS DEFINING A RESONANT REGION THEREIN FiledMarch 10, 1967 FIG. 3

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FIG. 5

INVENTORS JAMES M LIND JERRY C AUKLAND NW? A ATTORNEY MICROWAVE HARMONICGENERATOR INCLUD- INC A WAVEGUIDE HAVING OPPOSITELY EXTENDING CHANNELSDEFINING A RESO- NANT REGION THEREIN James N. Lind, Costa Mesa, andJerry C. Aukland, Fullerton, Calii'l, assignors to North AmericanRockwell Corporation, a corporation of Delaware Filed Mar. 10, 1967,Ser. No. 622,237

U.S. Cl. 32169 Claims Int. Cl. H03b 19/00; H03f 7/00 ABSTRACT OF THEDISCLOSURE A microwave harmonic generator for producing an output at afrequency nf (11:3, 4, 5 harmonically related to an input signal atfrequency f The device comprises a rectangular waveguide capable ofsupporting the input signal in the lowest order transverse mode andterminating in a second waveguide beyond cutoff. A pair of channelsextending oppositely from the top and bottom of the waveguide to a depthof one-quarter guide wavelength at (n1) define a waveguide pseudocavityregion resonant at (n1)j Interaction of the input signal with a varactordiode gives rise to energy within the psuedocavity region atharmonically related frequencies; this energy predominantly is at (nl))Parametric interaction occurs between the confined energy (at (n1)f andthe signal (at f to produce an output at n which may be extracted viathe second waveguide. A second pair of grooves oppositely extending fromthe top and bottom of the waveguide function as a trap to prevent energyat the output frequency from propagating back down the input waveguide.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to a microwave harmonic generator and more particularly to avaractor type harmonic generator utilizing a rectangular waveguidehaving a pseudocavity region therein which is resonant at a harmonic ofthe input frequency and terminating in an output waveguide whose cutofffrequency lies above the resonant frequency of the pseudocavity region.

Background of the invention As increasing effort is made to exploithigher and higher microwave frequencies for radar and other purposes,the need for providing RF sources at these extremely high frequenciesbecomes more important. Present RF sources, such as klystronoscillators, are limited in their very high frequency capabilities andare relatively inefficient at frequencies above about 15 gHz. Abovethese frequencies, new devices such as Gunn effect oscillators areuseful, but these have not yet been developed into operational unitsexcept at very low power levels. A more desirable way to provide RFfrequenceis above 15 gHZ. is to employ klystron or other oscillator at afrequency within its efficient operating range and to multiply thefrequency of the oscillator output using a harmonic generator.

In the past, a number of harmonic generators have been suggested. Forexample, the microwave varactor tripler described by C. B. Swan in theDigest of Technical Papers, 1965 International Solid State CircuitsConference (ISSCC), pp. 106407, utilizes a coaxial matching transformerto introduce an input signal to a varactor diode. The varactor diode isdisposed within an output waveguide which is tuned to the third harmonicof the input frequency and which also contains a transverse stubresonant at the second harmonic. A requirement of the nited StatesPatent 0 T 3,431,485 Patented Mar. 4, 1969 Swan device is that thevaractor package be resonant at the second harmonic. While this deviceprovides third harmonic output, the requirement that the varactorpackage be resonant at one of the harmonic frequencies inherently limitsthe flexibility of the device. When operated at extremely highfrequencies, this places severe restric tions on the varactor packagedimensions. (The described Swan device converted an input signal at 4gHz. to an output at 12 gHz.)

Another approach to harmonic generation which is not limited by theinherent characteristics of the varactor package is described by M. E.Hines and J. deKonig on pp. 22 and 23 of the 1967 ISSCC Digest ofTechnical Papers. This harmonic generator employs a snap varactor inwhich the capacitance is approximately constant over the useful range ofreverse bias. The device uses a cavity resonant at the fundamentalfrequency. A radial-line coupling gap couples the fundamental cavity toa second radial cavity resonant at the desired harmonic. A snap diode issituated within the second cavity. Coupling to an output coaxial lineoccurs via holes between the harmonic cavity and the coaxial outputline. Tenth harmonic output was reported for the Hines and DeKonigdevice to derive a 16 gHz. output signal using a 1.6 gHz. input. While aradial configuration is useful at these frequencies, it is verydifficult to construct such a radial device at much higher frequencies.

The present invention provides a harmonic generator of simple mechanicaldesign which is capable of operating with input signals above 50 gHz.The inventive harmonic generator is of rectangular configuration,thereby simplifying its construction, and while utilizing a varactordiode, its proper operation does not depend on the resonant frequency ofthe diode package.

SUMMARY OF THE INVENTION The inventive harmonic generator comprises arectangular waveguide having a width sufficient to allow propagation ofthe input signal in the lowest order transverse mode. The waveguideterminates in a second waveguide, the cutoff frequency of whichpreferably lies between output frequency nf (where n=3, 4, 5 and (nl)f Apair of channels extending oppositely from the top and bottom of theinput waveguide, and having a depth of one-quarter guide wavelength atfrequency (n1)f define a pseudocavity region within the input waveguide,which region is resonant at the (n1) harmonic of the input frequency t Avaractor diode is disposed within this pseudocavity region. The inputsignal, when impressed across the back-biased varactor diode, gives riseto energy at harmonically related frequencies. This results because ofthe nonlinear relationship of current to voltage exhibited by suchdiodes. Because the diode is disposed within a resonant cavity, theinteraction energy is concentrated at the resonant frequency (nl) and isconstrained to within the pseudocavity region. Parametric interactionoccurs between the constrained energy (at (nl)f and the input signal (at11,) to produce an output signal at the sum frequency nf The outputenergy, which may be extracted via the second waveguide, is preventedfrom escaping via the input waveguide by means of a trap comprising asecond pair of channels oppositely extending from the top and bottom ofthe input waveguide to a depth of one-quarter guide wavelength at nf Theinventive harmonic generator is easy to construct and, in a typicalapplication, may be used as a frequency tripler accepting an input at 60gHz. and providing an output at gHz.

Thus it is an object of the invention to provide a microwave harmonicgenerator of simple physical configuration.

Another object of the invention is to provide a harmonic generatorhaving an input waveguide which terminutes in a second waveguide beyondcutoff and which contains a pseudocavity region resonant at a harmonicof the input frequency.

It is another object of the invention to provide a simple microwaveharmonic generator utilizing a varactor diode to produce parametricinteraction between energy constrained at one harmonic frequency (n1)f11:3, 4, within a waveguide pseudocavity region with input energy at thefundamental frequency f to produce an output signal at the sum frequencynf A further object of this invention is to provide a microwave harmonicgenerator utilizing quarter wave channels to provide containment ofvarious harmonic signals to within various regions of a waveguide.

Other objects and features of the invention will become apparent fromthe following description and drawings which are utilized forillustrative purposes only.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged, perspectiveview of a preferred embodiment of the inventive microwave harmonicgenerator.

FIG. 2 is a cross sectional view of the harmonic generator as viewedgenerally along the line 22 of FIG. 1; a possible electric fielddistribution within the harmonic generator also is illustrated.

FIG. 3 is a cross sectional view of the inventive harmonic generator asviewed generally along the line 3-3 of FIG. 1.

FIG. 4 is a cross sectional view of the inventive harmonic generator asviewed generally along the lines 44 of FIG. 1.

FIG. 5 is another cross sectional view of the inventive harmonicgenerator as viewed generally along the line 5-5 of FIG. 1; a possibleelectric field distribution within this region of the harmonic generatoralso is illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there isshown a greatly enlargedperspective view of a preferred embodiment ofthe inventive microwave harmonic generator 10. For clarity, harmonicgenerator is illustrated as being of thin walled construction. However,because of the very small dimensions required for operation at highmicrowave frequencies, the device more easily may be constructed fromseveral solid blocks of metal which are appropriately milled andassembled to form a device having an interior appearance similar to thatshown in FIG. 1. In a typical embodiment having an input frequency ofabout 60 gHz., the overall length of harmonic generator 10 illustratedin FIG. 1 will be less than 0.5 inch.

The inventive microwave harmonic generator accepts an input signal at afundamental or first harmonic frequency f and provides an output signalat a frequency which is an integral multiple of the fundamental. Thus,the harmonic generator will provide an output at a frequency nf where11:3, 4, 5 For ease of exposition in portions of the followingdescription, the illustrative embodiment of FIG. 1 will be described interms of its use as a frequency tripler, providing a third harmonic(11:3) output. Throughout, mention will be made of how the variouscomponent parameters may be altered to allow generation of any otherdesired harmonic.

The input signal at fundamental frequency f is introduced into harmonicgenerator 10 via port 12 at one end of rectangular waveguide section 14.The other end of rectangular Waveguide section 14 terminates in shortingplane or wall 16. A second waveguide 40, having a width at and a heightb also extends from wall 16 and terminates in port 42. In a preferredembodiment, waveguide has a cutoff frequency which lies between thedesired output frequency inf and the frequency (n1)f of the next lowerharmonic. Thus, if harmonic generator 10 were to be used to triple aninput signal of frequency f :60 gHz., the third harmonic output at 3:180 gHz. may be extracted through waveguide 40. In this preferredembodiment, the cutoff frequency of waveguide 40 preferably is aboutgHz., midway between the second harmonic frequency (2f :l20 gHz.) andthe output frequency.

In a preferred embodiment, the interior width a (see FIG. 3) ofwaveguide 14 is selected to be one-half of the free space wavelength atthe fundamental frequency f Waveguide section 14 has a height b,measured between waveguide top 17 and bottom 18 (see FIG. 2), whichpreferably is less than width a. With these dimensions, it is possiblefor the input signal at frequency f to propagate in waveguide section 14in the lowest order transverse (TE mode.

As illustrated most clearly in FIG. 2, a first pair of channels 22 and22a extend oppositely from top 17 and bottom 18 of waveguide section 14.In a preferred embodiment, channels 22 and 22a have a depth d (see FIG.4) which is one-quarter of the guide wavelength at the frequency (nl)fwhere the desired output is at the r1 harmonic. (The function ofchannels 22 and 22a is somewhat similar to the U-shaped waveguidechannels described in the copending application to J. N. Lind et al.,Ser. No. 617,231, filed Feb. 20, 1967, entitled, Broadband MicrowaveParametric Amplifier, and assigned to North American Aviation, Inc.,assignee of the present application.) Thus, in the frequency triplerexample, the preferred depth d of channels 22 and 22a is one-quarter ofthe guide wavelength corresponding to Zf Side channels 24 and 24a (seeFIG. 1) are provided to achieve desired propagating modes for theharmonic energy present within regions of harmonic generator 10. In apreferred embodiment, channels 24 and 24:: have a depth c (see FIG. 5)equal to one-quarter of the Width of waveguide section 14 (i.e., c:u/4).That is, depth c corresponds approximately to one-eighth of thefree-space wavelength of a signal at f,,. Channels 24 and 24a, which mayextend beyond wall 16, further function to ensure suppression ofspurious modes within the harmonic generator.

Channels 26 and 260 function as a trap to prevent output energy at the nharmonic from propagating down rectangular waveguide section 14 towardport 12. In a preferred embodiment, the depth 1 (see FIG. 3) of channels26 and 26a is equal to one-quarter of the guide wavelength at outputfrequency nf Parameters for selection of the distance g between channels26 and 26a and diode 30 (see FIG. 2) are described hereinbelow.

A nonlinear reactance such as varactor diode 30 in a preferredembodiment is mounted within pseudocavity region 20 at a location to bedescribed in detail hereinbelow. Diode 30 may, for example, be of thetype described in the copending application to D. B. Anderson et al.,Ser. No. 361,069, entitled, High Frequency Diode," and assigned to NorthAmerican Aviation, Inc., assignee of the present application.Alternately, other varactor diodes may be employed. Diode 30 may beprovided with a cone-shaped contact 32, and electrical connections 34and 36, by means of which an appropriate backward bias voltage may beapplied to varactor 30.

In operation, an input signal at fundamental frequency f is introducedinto harmonic generator 10 via input port 12. This input signal,harmonics of which are to be generated, will be present throughoutwaveguide section 14. In a preferred embodiment, an impedance matehingnetwork (not shown in the figures) of a type well known to those skilledin the art may be used to insure that a minimum amount of energy at ,fis reflected back out of port 12 by harmonic generator 10. If the inputsignal at frequency f is introduced in the lowest order transverse mode(e.g., the TE mode) then the signal will not be propagated beyond wall16, since f is well below the cutoff frequency of output waveguidesection 40.

Within harmonic generator 10, when the input signal is impressed acrossback-biased varactor diode 30, energy at various harmonics of f will beproduced as a result of the nonlinear relation of current to voltage indiode 30. Thus energy will be present in rectangular waveguide section14 at (among others) the (n1) harmonic (where output is desired at the nharmonic). Since in a preferred embodiment channels 22 and 22a each havea depth of one-quarter guide wavelength at frequency (rt-1H the electricfield 50 induced in these channels at this frequency will be a maximumin the planes of the top 17 and bottom 18 of rectangular waveguide 14.Since this energy in effect is being generated at varactor diode 30, itwill start to propagate back along waveguide section 14. However, whenthis (nl) harmonic signal reaches the area between grooves 22 and 22a,it will see an extremely high impedance due to the electric field maximaat this location. As a result the (n1) harmonic signal will notpropagate further along waveguide 14 toward input port 12. Further,since the cutoff frequency of output waveguide 40 is higher than (n-1)fthis (n1) harmonic will not be propagated out of harmonic generator viaWaveguide 40. Thus energy at frequency (nl) will be constrained entirelyto within pseudocavity region 20.

If the length 1 (see FIG. 2) of pseudocavity region 20 is made to be anintegral number of one-half guide wavelengths plus one-quarter guidewavelength at the frequency (n1)f then pseudocavity region 20 willfunction as a resonant cavity at this (n-1) harmonic. In a preferredembodiment, the length 1 is selected to be three-quarters guidewavelength at the frequency of (n1)f Thus, when used as a frequencytripler, the electric field distribution within pseudocavity region 20at the second harmonic may have the appearance illustrated by solidarrows 50 in FIG. 2. In a direction perpendicular to the longitudinalaxis of waveguide section 14, the electric field at the frequency (nl),f=2 may, in a preferred embodiment, have the distribution shown by arrows52 in FIG. 5. Note that this distribution (which corresponds to a TEmode) results since the width a was selected to be one-half free spacewavelength, and the depth 0 selected to be one-eighth free spacewavelength at the fundamental frequency f Thus the dimension a will beslightly longer than one free space wavelength, and the dimension 0 willbe slightly longer than onequarter free space wavelength, at the secondharmonic 2].

With the preferred configuration described, pseudocavity region 20 ofharmonic generator 10 will exhibit a high Q at the frequency (nl)f Thuswhen a signal at input frequency i is introduced into waveguide section14, the harmonic energy generated by interaction with nonlinearreactance 30 will result in a preponderance of energy in the (rt-1)harmonic. For example, in the exemplary frequency tripler embodiment,the energy will be concentrated in the second harmonic, and may have thestanding wave pattern illustrated in FIGS. 2 and 5.

To produce an output at frequency nf harmonic generator 10 functions ina manner resembling a parametric up-converter. (A good theoreticalexposition of parametric interaction is contained in the text entitledCoupled Modes and Parametric Electronics, by William H. Louisell,published in 1960 by John Wiley and Sons, NY.) The signal, atfundamental frequency f,,, is introduced into waveguide section 14 viaport 12 and the pump, at a frequency (n1)f is induced into pseudocavityregion 20 as a result of the nonlinear current-voltage characteristicsof diode 30, as described hereinabove. (Note that when used as afrequency tripler, the device in effect is operating in the degenerateparametric mode, with the pump frequency 2 equal to twice the signalfrequency f.,.) In contrast to usual parametric device operation, energyis being fed to harmonic generator 10 only at the signal frequency, andnot also at the pump frequency. Rather, the pump energy is derivedcompletely within harmonic generator .10 from the signal. In a preferredembodiment, varactor diode 30 is situated at the pump voltage maximummidway between channels 24 and 24a (see FIG. 5) and one-half guidewavelength (at (nl)f from wall 16 (see FIG. 2). Parametric interactionoccurs between the signal at f and the pump at (n.l)f and results inproduction of energy at the idler frequency nf the sum of the pump andsignal frequencies. This energy, which will be present in pseudocavityregion 20, is at just the desired n harmonic.

The energy generated at the n harmonic may be extracted from harmonicgenerator 10 via waveguide 40, which, as noted above, has a cutofffrequency below nf In a preferred embodiment, an impedance matchingnetwork (of a type Well known to those skilled in the art), not shown inthe figures, may be provided between output port 42 and the load whichis to utilize the harmonic generator output signal. This impedancematching network will function to match the impedance of the load tothat of harmonic generator 10 and to minimize the amount of energy (atnf reflected back into the generator.

Channels 26. and 26a serve as a filter or trap to prevent energy at theoutput frequency nf from propagating out of harmonic generator 10 viawaveguide section 14. To perform this function, in a preferredembodiment channels 26 and 26a have a depth (see FIG. 3) of one-quarterguide wavelength at nf and are located at a distance g (see FIG. 2)which is an integral number of one-half guide wavelengths (at nf fromvaractor diode 30. Moreover, channels 26 and 26a preferably shouldextend from waveguide section 14 at a location between channels 22 and22a and input port 12, as illustrated in FIGS. 1 and 2.

In the exemplary harmonic tripler, channels 26 and 26a may be placed onewavelength (i.e. two half wavelengths) at 3f from varactor diode 30; inthis case, the electric field distribution at the output frequency mayhave the appearance illustrated by dashed arrows 56 in FIG. 2. Note thatthe third harmonic (output) energy will see a very high impedance in thearea of channels 26 and 26a, caused by the voltage maxima present inchannels 26 and 26a in the plane of waveguide top 17 and bottom 18. Thiswill effectively prevent energy at the output frequency from propagatingfurther down waveguide section 14 toward input port 12. Although notshown in FIG. 5, the tripler output energy at 3 f will extend into sidechannels 24 and 24a, and, in a preferred embodiment may be present inharmonic generator 10 in the TE mode. Thus the harmonic generator willproduce an output at frequency nf which will propagate down waveguide 40and be available at output port 42.

While the inventive harmonic generator has been described in terms of anembodiment having opposing pairs of channels 22 and 22a this is notrequired. Rather a single channel -(i.e., either 22 or 22a) extendingfrom either top 17 or bottom 18 of waveguide section 14 may suffice todefine pseudocavity region 20. However, when only one such channel isused, energy at the (rt-l) harmonic may not be constrained within region20 to the same degree as when opposing channels are used. This willresult in somewhat lower harmonic generator eificiency. In an alternateconfiguration, an additional channel or pair of channels may be addedbetween channels 22 and 22a and the output harmonic trap (channels 26and 26a) to form a second pseudocavity region, This will provideadditional containment of the energy at (n1)f,,, and may improve theconversion efficiency of harmonic generator 10.

Similarly, it may be possible to dispense with either channel 26 or 26aand still achieve sufiicient trapping of the output energy with theremaining single channel filter. On the other hand, an additional pairof channels,

one-quarter guide wavelength deep at the output frequency, may be added(between channels 26 and 26a and input port 12) to provide additionalattenuation of output energy propagating toward port 12.

We claim:

1. A harmonic generator comprising:

a rectangular waveguide capable of supporting energy at a fundamentalfrequency and terminating in a shorting plane,

a first pair of channels oppositely extending from the walls of saidwaveguide, said channels having a depth of onequarter guide wavelengthat a first frequency harmonically related to said fundamental frequency,and

a varactor diode disposed within said waveguide between said channelsand said shorting plane.

2. A harmonic generator as defined in claim 1 further comprising asecond pair of channels oppositely extending from the walls of saidwaveguide, said second pair of channels having a depth of one-quarterguide Wavelength at a second frequency, said second frequency beingharmonically related to said fundamental frequency and higher than saidfirst frequency, said second pair of channels extending the entire widthof said waveguide and being situated between said first pair of channelsand the nonshorted end of said waveguide.

3. A harmonic generator as defined in claim 2 further comprising asecond waveguide extending from said shorting plane and having a cutofffrequency between said first and second frequencies.

4. A- harmonic generator as defined in claim 3 wherein said first pairof channels extend the entire width of said waveguide, wherein saidwaveguide has a width equal to one-half free-space wavelength of saidfundamental frequency, and wherein said first pair of channels aresituated an odd number of one-quarter guide wavelengths at said firstfrequency from said shorting plane.

5. A harmonic generator as defined in claim 4 further including meansfor controlling the mode in which energy is present in regions of saidwaveguide, said means comprising first and second pairs of side channelsextending from said waveguide to a depth of approximately oneeighthfree-space Wavelength at said fundamental frequency, said pairs ofchannels being parallel to the longitudinal axis of said waveguide.

6. A harmonic generator for converting an input signal at a fundamentalfrequency f to an output at a harmonically related frequency 11f (where11:3, 4, 5 comprising, in combination:

a waveguide capable of supporting signal at frequency means comprisingat least one channel extending from the wall of said waveguide to adepth of essentially one-quarter guide wavelength at a frequency (rr-1)ffor providing a resonant region at frequency (11-1) within saidwaveguide, and

nonlinear reactance means, comprising a varactor diode disposed withinsaid region, for producing energy at (n1)1 and to provide an output atnf 7. A harmonic generator as defined in claim 6 further including trapmeans, comprising at least one channel extending from the wall of saidwaveguide to a depth of essentially one-quarter guide wavelength atfrequency nf for preventing energy at frequency nf from propagating downsaid waveguide beyond said trap means.

8. A harmonic generator as defined in claim 7 wherein said waveguideterminates in a second waveguide, said second waveguide having a cutoiffrequency between (11-1) and nf 9. A microwave harmonic generator forconverting an input signal at a frequency f to an output signal at afrequency nf (where n is an integer greater than 2), said generatorcomprising, in combination:

a first waveguide capable of supporting a signal at frequency f in thelowest order transverse mode and terminating in a shorting plane,

a second waveguide having a cutoff frequency between lif and (n1)f saidsecond Waveguide extending from said shorting plane,

means comprising at least one channel extending from said waveguide to adepth of one-quarter guide Wavelength at a frequency (nl)f for providinga psuedocavity region within said first waveguide, said region beingresonant at (nl)f a varactor diode disposed within said pseudocavityregion, and

trap means, comprising a second pair of channels oppositely extendingfrom said first waveguide to a depth of one-quarter guide wavelength ata frequency of nf for preventing energy at the output frequency frompropagating down said first waveguide beyond said trap means.

10. A microwave frequency tripler comprising, in combination:

a first waveguide capable of supporting a signal at a fundamentalfrequency in the lowest order transverse mode and terminating in ashorting plane,

a second waveguide having a cutoff frequency between the second andthird harmonics of said fundamental, said second waveguide extendingfrom said shorting plane,

a first pair of channels extending from said first waveguide to a depthof one-quarter guide wavelength at said second harmonic, said channelsextending the entire width of said waveguide and being locatedthree-quarters of a guide wavelength at said second harmonic from saidshorting plane,

a varactor diode disposed within said first waveguide at a locationequidistant from the sides of said waveguide and one-quarter guidewavelength at said second harmonic from said shorting plane, and

a second pair of channels oppositely extending from said first waveguideto a depth of one-quarter guide wavelength at said third harmonic, saidchannels extending the entire width of said waveguide and being locatedone guide wavelength at said third harmonic from said diode.

References Cited UNITED STATES PATENTS JOHN F. COUCH, Primary Examiner.

G. GOLDBERG, Assistant Examiner.

U.S. Cl. X.R. 330-49

